1. Field of the Invention
The invention is based on an exhaust gas sensor as generically defined in the main claim, and a method of producing the same.
2. Description of the Related Art
Exhaust gas sensors, particularly lambda sensors, are already known from, for example, DE-OS 2,265,309; these have a porous layer that covers the measuring electrode and is composed of a material that catalyzes the establishment of the gas equilibrium, which layer can also be, for example, a catalytically inactive, porous ceramic layer that contains finely-distributed precious metals. These precious metals, particularly from the platinum group, act as getters, that is, as collecting substances for pollutants from the exhaust gas that can be detrimental to the electrode function, for example lead, silicon, phosphorus, and zinc. At the same time, they catalyze the establishment of the thermodynamic equilibrium and thus fix the sensor control position close to the stoichiometric point, that is, at .mu.=1.
Furthermore, a number of elements of the periodic table and their oxides have already been proposed as getters for pollutants from the exhaust gas. For example, DE-OS 4,033,388 describes the use of mixed oxides consisting of at least one alkali metal oxide and one thermally stable oxide of an element having a valence of at least three, preferably from the IIIa, IIIb or IVb groups. However, getter substances on a nonprecious metal basis are insufficiently reactive, particularly at low and medium application temperatures between approximately 300.degree. and 600.degree. C. Within this temperature range the degree of pollution is especially high, because the pollutants can precipitate, whereas at high temperatures they can be removed with the measuring gas.
Therefore, for low and medium application temperatures highly-reactive getter substances on a precious metal basis must be used. The associated drawback of this is that the sensor control position is determined at the stoichiometric point, and can thus no longer be deliberately set.
It is an object of the invention to simultaneously ensure protection of the measuring element of an exhaust gas sensor against pollutants at low and medium temperatures, and to set the sensor control position in order to adapt it to the respective application, particularly to optimize engine power, fuel consumption, conversion rate of the catalytic converter, etc.